handle default properties in Dom

Assignment to default properties in QDS are automatically merged with the children.
The Dom distinguishes children and bindings (the name of the default property can be found only when loading the dependencies, and the Dom is supposed to be usable even without dependencies.
Thus when mapping to the QDS Model children and possible bindings to the defaultPropertyName should be explicitly merged.
The peculiar relationship between the data/childrens/resources properties should also be handled explicitly, the Dom does not do any magic on them.